When riding any form of mass transportation there are unwritten rules:
1) Keep your eyes to yourself. ( I find a book helpful here)
2) Don't sit on a seat that is facing backwards. (Just weird)
3) Keep a straight face. (This applies specifically to situations when someone sits near you who hasn't showered but has tried to hid the stench with cheap perfume)
4) Don't sit next to anyone unless you can't find another empty seat. (This rules is an important one)
5) Ignore people acting strangely, crying children, and stupid schoolchildren without parental supervision. (It just feeds the insanity).
6) Be conscious of the elderly and offer them you seat. (this is just good manners even though most people don't do this.)